1. Field of the Invention
The present invention is directed toward the field of filters, and more particularly toward image rejection notch filters.
2. Art Background
Typically, receivers employ filters to condition both input signals and internally generated reference signals. For example, bandpass, notch, and low pass are types of filters employed in receivers. The frequency response of a filter refers to the characteristics of the filter that condition the signal input to the filter. For example, a bandpass filter may attenuate an input signal across a pre-determined band of frequencies above and below a center frequency of the filter. Filters are designed to exhibit frequency responses based on one or more circuit parameters.
Some receivers are designed to process input signals with a range of input carrier frequencies (e.g., wide band receivers). For example, television receivers must be capable of processing input television signals with carrier frequencies ranging from 55 MHz to 880 MHz. One circuit parameter used to define the frequency response of a filter is the carrier frequency of an input signal.
FIG. 1 illustrates one embodiment for an image rejection mixer, including a resistive-capacitive (“RC”) filter output. As shown in FIG. 1, a signal is input (e.g., RF input) to the in-phase (“I”) mixer 110 and the quadrature phase (“Q”) mixer 120. Also input to the (“I”) mixer 110 and quadrature phase (“Q”) mixer 120, at the local oscillator (“LO”) port, is a local oscillator signal.
As shown in FIG. 1, the I and Q signals are input to the resistor 140 and capacitor 130, respectively. The transfer function of the RC filter may be expressed as:
      A    =                            1          +                                    (                              -                j                            )                        ×            S                                    1          +          S                    =                        (                      1            +            Z                    )                          (                      1            +            S                    )                          where    ,                  ⁢          S      =              j        ⁢                                  ⁢        wCr                  S    =                  j        ×        Z            =              j        ×                  W          /          Wo                    
FIG. 2 illustrates a frequency response for the prior art image rejection mixer of FIG. 1. The frequency response of FIG. 2 is normalized. The image frequency, at −1, is attenuated and the desired frequency range is part of the passband of the filter. The transfer function, for the normalized response, may be expressed as:
      (          1      +      X        )              1      +              X        2            
Conventionally an image rejection mixer applies a single notch in the baseband after demodulation. When the desired signal has a relatively wide bandwidth, the image rejecting single notch can remove only one image frequency point. In other words, the single notch cannot remove a desired band. For this reason, the conventional image rejection mixer requires multiple stages of down conversion.